Backup power supplies are often used to supply power when a utility power grid is disabled. Internal combustion engine driven generators are often deployed as backup power supplies. These generators, however, are relatively large and noisy, and output toxic emissions.
Fuel cells are known to generate electrical power through chemical processes having relatively minimal emissions with little environmental impact. Accordingly, fuel cells have been explored as an attractive alternative to conventional backup generators. Unlike generators, however, fuel cells typically cannot be activated within a short period of time. Accordingly, in backup power applications, fuel cells often output electrical power continuously, regardless of whether the power grid is operational. In the event power is cut off, the fuel cell supplies backup power which can be distributed by the grid to critical components, or selected areas of the grid.
U.S. Pat. No. 6,700,804 describes a backup fuel cell coupled to a utility grid through a power converter. The power converter includes an inverter, which receives a direct current (DC) input voltage from the fuel cell, and outputs an AC signal to a utility distribution system.
In certain applications, however, a critical load, such as a bank of computers or a building housing sensitive equipment, is connected to the power converter in addition to the utility power grid. During normal operation, the utility power grid and the power converter supply AC power to the critical load. The current output from the power converter is regulated in a known manner, and the critical load voltage is set to the utility power grid voltage. When a fault occurs in the utility power grid, however, the power converter alone supplies AC power to the critical load. Although, the current is controlled, the voltage associated with the AC power is not, and may depend upon the level of loading of the critical load. Thus, the voltage associated with the AC signal output from the power converter can spike and cause damage to the critical load, or fall below levels required by the critical load.
The present disclosure is directed to overcome one or more of the shortcomings in the prior art.